Determination of the optimal discharge pressure of the transcritical CO2 heat pump cycles for heating and cooling performances based on new correlation

Considering the expansion of heat pump systems as one of the environment-friendly cycles, increasing these cycles' efficiency is significant. This paper is a comprehensive study of transcritical CO2 heat pump cycles in heating and cooling modes covering a range of ambient temperatures from -15 to 30 degrees C along with the range of 25 to 45 degrees C in cases of gas-cooler exit temperatures. In this study, the ambient temperature of 0 degrees C and gas-cooler exit temperatures of 30 and 35 degrees C were examined, and the best Coefficient of Performance and exergy efficiency were obtained at a discharge pressure of 7496 and 8896 kPa, respectively. The ambient temperature of 30 degrees C and gas-cooler exit temperatures of 45 and 50 degrees C were investigated, and the discharge pressure of 10,938 and 12,363 kPa have the highest Coefficient of Performance and exergy efficiency, respectively. The results show that the gas-cooler temperature positively affects the optimum discharge pressure. In conclusion, an equation for Optimum Discharge Pressure is presented as a general correlation to obtain this parameter for the heating and cooling modes in these commonly used cycles. This equation also presented with a deviation less than 5.66% of the optimum discharge pressure obtained from energy and exergy analysis. Therefore, it has been identified as an acceptable criterion in these cycles. Furthermore, by examining the amount of exergy destruction of these cycles' most essential components, it becomes clear that they are the most exergy destruction related to the compressor.

Automated summary

This paper presents a comprehensive study on transcritical CO2 heat pump cycles, analyzing the influence of different temperature conditions on performance. It introduces an equation to determine the Optimum Discharge Pressure with high accuracy in commonly used cycles, and highlights the impact of gas-cooler temperatures on performance.